Many of those in Australia, parts of California, and other regions of the world that experience wildfires, grapes that are exposed to the smoke of these fires often possess whatâ€™s referred to as â€śsmoke taintâ€ť: a highly undesirable smoky, ashy, or overall burned sensory characteristics that drive quality as well as price downward.Â In order to test for smoke taint in wines, guaiacol and 4-methylguaiacol levels are often analyzed, though the downside to this method is that itâ€™s not always reliable.Â For example, oak barrels or chips will also impart guaiacol and 4-methylguaiacol into the wine, so simply finding these compounds in your wine doesnâ€™t necessarily mean it suffers from smoke taintâ€”it may have just been aged in an oak barrel!Â Also, sometimes very little guaiacol is measured in the wine, when in fact later on smoke taint characteristics appeared as if from nowhere.
As a result of this inconsistency, there is a push to try and find better biochemical markers for smoke taint in wine, which will make diagnostic testing of grapes and/or wine so much faster and cost-effective, for if a reliable test for smoke taintis developed, one could test the grapes right at harvest and not bother to make wine from those berries if, in fact, the smoke taint markers were present.
Past studies have indicated that after a grape takes up the smoke-derived volatile compounds, they undergo a chemical reaction resulting in what are known as â€śphenolic glycosidesâ€ť, some of which have been identified (Î˛-D-glucosyl-Î˛-D-glucosides, Î˛-D-glucopyranosides, pentosylglucosides, and rutinosides).Â Basically what happens is that the grapes take up these smoke-derived volatiles and then convert them into more stable glycoside versions of the compounds.Â Also, the greater the smoke exposure, the greater the levels of smoke-derived volatiles and stable glycosides.Â Â These glycosides act as â€śprecursorsâ€ť to the smoke-derived volatile compounds, so if you have grapes that have been exposed to smoke and thus high levels of smoke-derived volatiles and their glycoside precursors, your wine is at risk for developing undesirable aromatic characteristics not only in the beginning of the winemaking process, but also through aging and storage.
The goal of the study today was to find another way to measure smoke taint in grapes and wine other than the current method of measuring guaiacol and 4-methyguaiacol.Â Specifically, the study aimed to use smoke-derived stable phenolic glycosides as smoke taint markers, in hopes to find a quick and reliable method for winemakers to measure potential smoke contamination in grapes.
Chardonnay and Shiraz grapes that were exposed to the smoke from bushfires in Victoria, Australia in 2009 were harvested for analysis, as well as control grapes from other vineyards around Australia that had not been exposed to smoke.
Chardonnay and Cabernet Sauvignon wines that were previously created from grapes that were exposed to smoke were also analyzed.
Phenolic glycosides were analyzed using HPLC-MS/MS techniques, and guaiacol was analyzed using GC-MS techniques.
- The techniques used for both the red and white grapes/wine seemed efficient and accurate in analyzing phenolic glycosides, and was also repeatable (a good quality to have!).
- Phenolic glycoside levels were apparently higher in smoke-exposed grapes and wine than levels of guaiacol.
- There was a relatively wide range of glycoside levels in each grape variety, which may be due to the position of the grape on the vine, and how exposed to the smoke it was while still on the vine (this would need to be tested).
- The most abundant glycoside in all varieties/varietals was Syringol gentiobioside, 5, then 6, and finally the pentosylglucosides and rutinosides.
- Individual glycoside levels were well correlated with total glycoside levels in grapes and wine.
- Results indicated that smoked-derived volatile compounds, particularly their phenolic glycoside precursors, may be good markers for measuring smoke taint in grapes and wine.
- Guaiacol levels in many of the known smoke-tainted grapes and wine were within the range of the control grapes/wine, which were not exposed to smoke.
- Analysis indicated that measuring guaiacol alone was not sufficient to identify a wine as exposed to smoke or not, while it was clear from measuring phenolic glycosides which grapes/wines were exposed to smoke and which were not.
- Using guaiacol as a marker, only 3 out of the 6 wines that were exposed to smoke taint were identified as such, while when using phenolic glycosides, the analysis was able to identify all 6 wines as being smoke-tainted.
- The two best phenolic glycosides for identifying smoke taint in grapes and wines were Syringol gentiobioside 5 and Syringol gentiobioside 6.
The results of this study were significant in that it was determined that using phenolic glycosides, specifically Syringol gentiobioside 5 and Syringol gentiobioside 6 as markers for smoke taint in wine is much more effective and accurate than measuring guaiacol levels.Â According to the authors of this study, these findings indicate a vast improvement over current smoke taint analysis measurement techniques, and using phenolic glycosides may in fact be more accurate as well as time efficient.
It is important to note that this study was just a pilot study: meaning the sample sizes were relatively low.Â It would be very interesting to see this study done on a large scale, incorporating many different grape varieties both exposed and notexposed to smoke, and from different areas of the world.Â According to the authors of this study, they are currently performing such a study, so hopefully once that is finished and the results are published, I can bring you more convincing (or not!) results.Â Â These future studies will also be examining improving the reliability of the analysis, as well as specific recommendations for analytical methods that one could use in the laboratory to test grapes for smoke taint exposure prior to going through the winemaking process.
What do you all think about this study as well as this new technique?Â Do you have any experience in this field?Â Do you have any other ideas on how to more accurately test for smoke taint at any early stage so that one does not waste the time and resources on creating the wine before one realizes itâ€™s contaminated?Â Please feel free to leave any comments you have!
Source: Hayasaka, Y., Parker, M., Baldock, G.A., Pardon, K.H., Black, C.A., Jeffery, D.W., and Herderich, M.J. 2013. Assessing the impact of smoke exposure in grapes: Development and validation of a HPLC-MS/MS method for the quantitative analysis of smoke-derived phenolic glycosides in grapes and wine. Journal of Agricultural and Food Chemistry 61: 25-33.